1. Field of the Invention
The present invention relates to a defect classifying method and an inspection apparatus that detect a defect which is present at a silicon carbide substrate (SiC substrate) and classify the detected defect.
2. Description of Related Art
Silicon carbide has superior physical and thermal properties, and thus is useful for production of a high-voltage, low-loss semiconductor device. In a semiconductor device production process using a SiC substrate, detection of defects which are present at the SiC substrate and classification of the defects which are detected are extremely important with regard to improving the production yield. Especially, in the case of producing a bipolar high-voltage element, a basal plane dislocation (BPD) is a killer defect which has an adverse effect on the performance of the element. For this reason, there is a strong demand for detecting defects by distinguishing the BPD from other defects.
A structure conversion technique for converting basal plane dislocations to threading edge dislocations (LEDs) in the process of forming an epitaxial layer during a SiC substrate production process has been developed. Specifically, in the case where a bipolar device is formed in an area including the BPD, when a current flows in the forward direction, the BPD is changed to a Shockley-type stacking fault, which causes a change in device characteristics and leads to a malfunction. On the other hand, when a bipolar device is formed in a location where the TED is present, the device characteristics are less affected. For this reason, devices that can be put to practical use by using a substrate including the TED can be produced. In addition, the use of the above-mentioned structure conversion technique makes it possible to convert most BPDs existing in the SiC substrate into TEDs. This leads to a further improvement in the production yield of the devices and is extremely useful for the production of high-voltage elements.
On the other hand, the structure conversion from BPDs to TEDs is carried out by appropriately controlling various parameters for metal organic chemical vapor deposition (MOCVD) in the epitaxial layer growth process. To find out the optimum conditions for various parameters in the MOCVD process, it is important to detect BPDs which are present at the SiC substrate on which an epitaxial layer is formed. In other words, if BPDs can be detected at the SiC substrate on which an epitaxial layer is formed, information useful for controlling the MOCVD process can be fed back. Accordingly, in order to improve the efficiency of the conversion from BPDs to TEDs and improve the production yield, it is necessary to develop a defect inspection apparatus capable of selectively detecting defects by distinguishing a basal plane dislocation from other defects.
As an apparatus for inspecting a SiC substrate, an inspection apparatus using a differential interference optical system is known (see, for example, Japanese Unexamined Patent Application Publication No. 2012-174896). In this inspection apparatus, a line-shaped scan beam is projected toward the surface of the SiC substrate through the differential interference optical system and an objective lens. A line sensor receives reflected light reflected on the surface of the SiC substrate. The inspection apparatus that uses the differential interference optical system can detect a change in unevenness of about several nm on the surface of the SiC substrate as a contrast image. This is advantageous in that minute defects can be detected in the image of the surface of the SiC substrate.
As another inspection apparatus, an inspection apparatus that uses a photoluminescence method (PL method) is known (for example, see Japanese Unexamined Patent Application Publication No. 2006-147848). In this known inspection apparatus, ultraviolet light is projected toward a silicon carbide substrate. Further, a photodetector detects photoluminescence light emitted from the silicon carbide substrate through a spectrometer. This known inspection apparatus detects the photoluminescence light, which is advantageous in that a crystal defect existing in the substrate can be detected.
The inspection apparatus that uses the differential interference optical system can detect a minute change in the unevenness on the surface of the substrate as a luminance image. Accordingly, threading screw dislocations and threading edge dislocations, which appear as a pit structure on the surface of the substrate, can be detected. However, the basal plane dislocations hardly appear on the surface of the substrate, or rarely appear as a minute pit structure. Accordingly, the inspection apparatus that uses the differential interference optical system has a drawback that the inspection apparatus cannot clearly detect the basal plane dislocations. On the other hand, in the inspection apparatus that uses the photoluminescence method, a defect existing in the substrate can be detected in a PL image, which is advantageous in that the basal plane dislocations can be clearly detected.
On the PL image, a location where the basal plane dislocation exists is detected as a line-shaped PL image. Accordingly, the basal plane dislocation can be detected in the PL image. However, a carrot defect also exists as a crystal defect specific to the epitaxial layer formed on the SiC substrate. A carrot defect is also a killer defect in the device production process, and thus the carrot defect needs to be detected in the defect inspection process. The PL image of a carrot defect is a line-shaped image which is similar to that of the basal plane dislocation. Accordingly, the carrot defect can be detected by a photoluminescence light inspection. However, the cause of occurrence of the carrot defect and the measures to prevent the occurrence of the carrot defect are different from those of the BPD. Therefore, there is a strong demand for detecting the BPD and the carrot defect by distinguishing them from each other. However, since both the PL image of the BPD and the carrot defect are line-shaped images, it is actually difficult to detect the BPD and the carrot defect by distinguishing them from each other in the photoluminescence light inspection.
There is also a strong demand for further increasing the accuracy of classifying detected defects in order to improve the production yield in the device production process using the SiC substrate. In other words, if the accuracy of classifying defects can be further improved, information useful for the epitaxial layer growth process can be fed back. Accordingly, in order to improve the production yield of devices that uses silicon carbide, it is also important to further improve the accuracy of classifying defects.
An object of the present invention is to realize a defect classifying method and an inspection apparatus which are capable of classifying defects by distinguishing a basal plane dislocation, which is a killer defect in bipolar high-voltage elements, from other defects. Another object of the present invention is to realize a defect classifying method and device capable of classifying defects, which are present at a SiC substrate, with high accuracy.